# Aluminium phosphate

Aluminium phosphate is a chemical compound. In nature it occurs as the mineral berlinite. Many synthetic forms of aluminium phosphate are known. They have framework structures similar to zeolites and some are used as catalysts, ion-exchangers or molecular sieves. Commercial aluminium phosphate gel is available.

## Berlinite

AlPO$_4$ is isoelectronic with Si$_2$O$_4$, silicon dioxide. Berlinite looks like quartz and has a structure that is similar to quartz with silicon replaced by Al and P. The AlO$_4$ and PO$_4$ tetrahedra alternate. Like quartz, AlPO$_4$ exhibits chirality and piezoelectric properties. When heated, crystalline AlPO$_4$ (berlinite) converts to tridymite and cristobalite forms, and this mirrors the behaviour of silicon dioxide.

## Uses
### Molecular sieves

There are many types of aluminium phosphate molecular sieves, generically known as "ALPOs". The first ones were reported in 1982. They all share the same chemical composition of AlPO$_4$ and have framework structures with microporous cavities. The frameworks are made up of alternating AlO$_4$ and PO$_4$ tetrahedra. The denser cavity-less crystalline berlinite, shares the same alternating AlO$_4$ and PO$_4$ tetrahedra. The aluminophosphate framework structures vary one from another in the orientation of the AlO$_4$ tetrahedra and PO$_4$ tetrahedra to form different-sized cavities, and in this respect they are similar to the aluminosilicate zeolites, which differ in having electrically charged frameworks. A typical preparation of an aluminophosphate involves the hydrothermal reaction of phosphoric acid and aluminium in the form of hydroxide, an aluminium salt such as aluminium nitrate salt or alkoxide under controlled pH in the presence of organic amines. These organic molecules act as templates (now termed structure directing agents, SDAs) to direct the growth of the porous framework.

### Other

Along with aluminium hydroxide, aluminium phosphate is one of the most common immunologic adjuvants (efficiency enhancers) in vaccinations. Aluminium adjuvant use is widespread due to its cheap price, long history of use, safety and efficiency with most antigens.
Similar to aluminium hydroxide, AlPO$_4$ is used as an antacid. Up to 20% of aluminium from ingested antacid salts can be absorbed from the gastrointestinal tract – despite some unverified concerns about the neurological effects of aluminium, aluminium phosphate and hydroxide salts are thought to be safe as antacids in normal use, even during pregnancy and breastfeeding.
Additional uses for AlPO$_4$ in combination with or without other compounds are white colorants for pigments, corrosion inhibitors, cements and dental cements. Related compounds have also similar uses. For example, Al(H$_2$PO$_4$)$_3$ is used in dental cements, metal coatings, glaze compositions and refractory binders; and Al(H$_2$PO$_4$)(HPO$_4$) is used cement and refractory binders and adhesives.

## Related compounds

AlPO$_4$·2H$_2$O dihydrate is found as the minerals variscite and meta-variscite. Aluminium phosphate dihydrate (variscite and meta-variscite) has a structure that can be regarded as an assembly of tetra- and octahedral units of phosphate anions, aluminium cations and water. Al$^{3+}$ ions are 6-coordinate and PO$_4$$^{3-}$ ions are 4-coordinate.
A synthetic hydrated form, AlPO$_4$·1.5H$_2$O is also known.